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Despite waning efficacy, influenza vaccine protects throughout season
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Vaccination appeared to remain efficacious throughout an influenza season, despite a modest decrease of the inactivated influenza vaccine’s efficacy over time, according to data recently published in The Journal of Infectious Diseases.
“Influenza vaccines are now available from some providers as early as July, and influenza activity may continue into the late spring,” Joshua G. Petrie, MPH, of the department of epidemiology at the University of Michigan School of Public Health, and colleagues wrote. “As a result, there can be relatively long periods between vaccination and potential exposure, raising concerns about the possibility of waning vaccine efficacy over a single season.”
Joshua G. Petrie
Although the Advisory Committee for Immunization Practices recommends influenza vaccination at the earliest opportunity, data suggesting waning protection over the course of a season have raised concern, the researchers wrote. To investigate this concern, Petrie and colleagues re-examined data from a randomized, controlled clinical efficacy trial conducted during the 2007-2008 influenza seasons.
As previously reported, healthy adults were recruited to receive either inactivated influenza vaccine (IIV), live-attenuated influenza vaccine (LAIV) or placebo in October and November, and were instructed to report any acute respiratory illnesses throughout the influenza season. Blood specimens were collected immediately before vaccination, 30 days after vaccination and at the end of the influenza season and tested to determine relevant antibody titers. Throat swabs collected upon illness visits were tested by real-time reverse transcription–PCR to confirm influenza infection. To measure for waning efficacy, Petrie and colleagues employed a Cox proportional hazard model that included a coefficient to estimate time-varying efficacy (VE[t]) among the three patient cohorts.
The mean age among the study’s 1,952 participants was 23 years, with no demographic variation between study groups. The 2007-2008 influenza season was dominated by influenza A(H3N2), which circulated for 16 weeks from early January through mid-April. There were 119 participants infected, of which 106 infections were attributed to influenza A(H3N2).
Cumulative incidence was 3.4% among IIV recipients, 6.9% among LAIV recipients and 10.8% among controls, translating to overall efficacy scores of 70% (95% CI, 50-82) for IIV and 38% (95% CI, 5-59) for LAIV. While the data suggested a decrease in IIV efficacy (P = .03) and antibody titers over time, both vaccine types remained efficacious throughout the season.
“While waning of IIV efficacy was detected in analyses considering all influenza viruses and influenza A(H3N2) virus specifically, the vaccine maintained a high level of protection for the majority of the influenza season in subjects vaccinated approximately 3 months prior to peak influenza activity,” Petrie and colleagues wrote. “The degree of waning efficacy observed here would not suggest an overall benefit to delaying vaccination and supports current recommendations for receipt of influenza vaccine at the earliest opportunity each season.”
Kathleen M. Neuzil
In a related editorial, Infectious Disease News Editorial Board member Kathleen M. Neuzil, MD, MPH, director at the Center for Vaccine Development, University of Maryland School of Medicine, lamented the study’s focus on healthy young adults, who often are not as affected by seasonal influenza as younger children or aged adults. Regardless, she wrote that the study’s detection of waning — but still present — influenza A(H3N2) vaccine efficacy is relevant to ongoing discussions on optimal populationwide vaccine coverage.
“Influenza vaccination policy must take into account the uncertainty of influenza virus circulation and the need for consistency to facilitate planning,” she wrote. “The trade-off is that, to vaccinate as many people as possible, in some seasons there may be modest declines in efficacy over time. Future studies of influenza vaccine effectiveness, including that of the newer vaccines, should incorporate the variable reflecting the time of vaccination relative to the time of influenza virus circulation into their analyses.” – by Dave Muoio
Disclosures: Petrie reports grant support from Sanofi Pasteur. Neuzil reports no relevant financial disclosures. Please see the full study for a list of all other authors’ relevant financial disclosures.
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